The present invention relates to a moving-picture coding apparatus, method and program, and a moving-picture decoding apparatus, method and program, with a boundary condition obtained between moving-picture blocks and applied for coding or decoding.
Several types of video broadcast services via digital broadcasts or networks are popular now. This creates a demand for higher moving-picture coding efficiency for recording a larger amount of moving pictures at a higher picture quality and definition.
There are now several new coding methods, such as, MPEG-4 AVC (which will be referred to as AVC or H. 264, hereinafter) with drastically higher coding efficiency than known coding methods. These new coding methods promote more efficient data encoding. The methods perform video-data compression with combination of MC (Motion Compensation) and DCT (Discrete Cosine Transform).
Known MC is conducted for each rectangular zone (typically, a square block which will be referred to as an MC block or simply a block, hereinafter), to detect the degree of motion based on the assumption that all of the pixels in each rectangular zone exhibit the same motion, thus generating motion-vector data. Thus, there is a case in that a video signal is discontinuous on an MC-block border in a predictive picture constituted by spatially arranged MC blocks. Such a discontinuous situation on the MC-block border results from per-block coding procedures, such as, detection of the degree of motion followed by orthogonal transform, quantization, etc. This phenomenon becomes more remarkable as the amount of data for use in coding becomes smaller, which is mostly noticeable as block distortion.
Especially, discontinuity on a block border and mismatching of texture data in block are mostly caused by a larger motion of an object in a moving picture, deformation of the object itself, variation in relativity among several objects, i.e., disappearance or appearance of the objects, etc., between pictures of moving objects.
A method to remedy the discontinuous situation on an MC-block border discussed above is disclosed, for example, in International Publication No. WO2003/003749A1. In this method, a procedure to adaptively smooth a discontinuous waveform generated on a border of MC blocks in a predictive picture produced by per-block motion compensation is performed by a relatively easy operation with a smoothing filter provided following to a motion compensator. This enhances coding efficiency at a low bit rate with motion compensation.
The above method applies smoothing to a video signal at pixels adjacent to a border of blocks in aiming for mitigating a discontinuous situation in order to remedy the discontinuous situation between blocks and mismatching of texture data. The discontinuous situation between blocks may, however, not be mitigated enough, which depends on the degree of smoothing.
For example, a lower degree of smoothing than required cannot mitigate enough a discontinuous situation between blocks which thus remains as block distortion. In contrast, a higher degree of smoothing than required can mitigate the discontinuous situation between blocks, which is, nevertheless, too much for texture data in block, thus resulting in low quality in the texture data.
Smoothing is followed by orthogonal transform, quantization and entropy coding to a residual video frame produced by subtraction between a reference picture and a moving-picture frame to be coded. Smoothing is applied to a predictive picture produced by motion compensation after an optimum block is selected in motion estimation, thus such a block may not always be optimum after this procedure. There may be several blocks having the same quality as that block in the predictive picture after smoothing. Thus, it requires a larger amount of computation for obtaining more appropriate motion-vector data under consideration of the code amount of motion-vector data and orthogonal-transform coefficients data after quantization, because of repeated operations of motion estimation, motion compensation and smoothing procedures for obtaining optimum motion-vector data.